The present invention relates to a method of mounting a printing plate to a mounting sheet, the mounting sheet facilitating subsequent mounting of the plate to a printing apparatus such as a printing roller.
The process of mounting printing plates to a backing sheet is known as pre-mounting. In the packaging production industry, for example, in which pre-mounted printing sheets are produced to enable a printer to print images onto packaging, such as for instance cardboard boxes, the established method of producing and pre-mounting printing plates is as follows.
The original artwork is first produced on a computer using a suitable CAD software package. Typically the artwork will comprise a number of multi-coloured designs for printing on different panels of a cardboard box. Once finalised, the computer panel design data is supplied to a printer for the production of full sized negative prints of the panel designs which are subsequently used to produce the printing plates. Since the printing process requires a separate set of plates for each colour to be printed, at the artwork production stage each different colour used in the panel designs is flagged as such by the software so that separate sets of negative prints can be produced for each individual colour.
The negative prints are then used to produce relief printing plates for each panel design from photo-polymer. These plates are then pre-mounted before sending to the printer. The plates for each individual colour are mounted on separate backing sheets which are generally all of the same size. The backing sheets are of a standard size and are provided with register holes or other alignment formations for fixing in a predetermined position on the printing rollers. The backing sheets and rollers generally have centre marks so that the printer can accurately align the separate backing sheets carrying the different colour plates.
It will be appreciated that although the printer can align the separate sheets with one another, the plates for each colour must be accurately positioned on respective backing sheets at the pre-mounting stage so that at the printing stage they are correctly positioned relative to one another once the backing sheets are aligned. For instance if each sheet carries a single plate each plate must be positioned at the identical position on its respective sheet to the position of the other plates on their backing sheets.
Such accurate pre-mounting is achieved as follows. The first sheet is placed on a laying up board in a set position for instance by locating one corner of the sheet within a right angle. This ensures that successive sheets can be positioned on the board in the identical position.
The dimensions of the cardboard blank, for instance, which the design is to be printed on is fed into a plotter which produces a full size plot of the blank on a sheet of tracing paper. This sheet of tracing paper is laid over the backing sheet and secured against unwanted movement by taping one edge of the sheet to the board.
The printing plate or plates of the first colour are then slipped under the tracing paper and fixed to the backing sheet. The correct position of the first colour plate or plates is determined by eye with reference to the outline of the blank carried by the tracing paper. Register marks are then made on the tracing paper to indicate the positioning of the first colour plate or plates.
The first backing sheet is then successively replaced with subsequent backing sheets of identical size and which are positioned on the board in the same position as the first in the manner discussed above. The tracing paper being taped to the board will remain in the same position. Subsequent colour plates are then fixed to respective backing sheets, the register marks made on the tracing paper after the first colour was mounted being used to help correctly align each colours plates with the first colours plates and with each other. For instance if an identical number and size of plates is used for each colour the mounting of the second and subsequent colour plates will be a simple matter of aligning those plates exactly with the register marks made after the first colour plates are mounted. It will however be appreciated that great care must be employed by a skilled worker in positioning the first colour plates as this is done by eye with reference only to the outline of the blank.
In addition, any single colour will often only appear at distinct spaced apart portions of any given panel design. In this case, since the cost of the photo-resin is high, it is not economical to produce a single printing plate for each colour for each panel design but rather a number of smaller plates are produced for respective localised areas of any one colour. Thus in general there will be a number of separate plates for each colour of any one panel design. This complicates the pre-mounting task since each plate of the first colour must be positioned accurately, by eye, relative to each other as well as to the outline of the blank.
Moreover, there will often be different numbers of different sized plates for each colour and therefore whilst the register marks of the first colour plates will aid in positioning subsequent colour plates, the positions of the subsequent colour plates may not correspond exactly with the register marks.
Since as many as 100 plates may be used for all the colours for any one blank it will be appreciated that the established method of pre-mounting is difficult and time consuming requiring painstaking work by a skilled worker. Thus even if the design consists of only a single colour the process of pre-mounting the plates in the correct relative positions by eye alone can be a difficult and time consuming procedure.
Plate mounting machines have been developed for pre-mounting, such as that sold by J. M. Heaford Limited under the trade mark VIPER, which employ video cameras to help position the plates. However, such machines are generally expensive, typically costing tens of thousands of pounds, and the number of plates which can be mounted is limited. For instance, the VIPER (trade mark) machine referred to can be used to mount a maximum of twelve plates only.
It is an object of the present invention to obviate or mitigate the above disadvantages.